As a recording medium for recording or reproducing digital audio data, a mini disc (MD) as a magnetooptic disc having a diameter of 64 mm and enclosed in a cartridge has widely been spread.
In an MD system, ATRAC (Adaptive TRansform Acoustic Coding) is used as a compression system of the audio data. ATRAC is a technique such that the audio data fetched at a predetermined time window is compression-coded by using MDTC (Modified Discrete Cosine Transform). Music data is compressed into ⅕ to 1/10 by ATRAC.
A convolutional code called ACIRC (Advanced Cross Interleave Reed-Solomon Code) is used as an error correction system and EFM (8 to 14 Modulation) is used as a modulation system. ACIRC is a convolutional code for executing error correction coding doubly to a C1 series (vertical direction) and in an oblique direction (C2 series) and a powerful error correcting process can be executed to sequential data such as audio data. In the case of the convolutional code, however, a sector for linking is necessary at the time of rewriting the data. ACIRC system and EFM similar to those for the existing compact disc (CD) are fundamentally used.
U-TOC (User TOC (Table Of Contents)) is used for management of the music data. That is, an area called U-TOC is provided for an inner rim of a recordable area on the disc. In the existing MD system, U-TOC is management information which is rewritten in accordance with order of music pieces, recording, erasure, or the like of tracks (audio tracks/data tracks) and manages a start position, an end position, and a mode with respect to each track (parts constructing the track).
The disc of the MD system is small and low-priced and has excellent characteristics as a medium for recording and reproducing the audio data. The MD system, therefore, has widely been spread.
However, some improvements have been needed in the existing MD system as a result of the passage of time. One of them is affinity with computers.
That is, music distribution for distributing the audio data on a network has extensively been performed owing to the spread of personal computers and the networks. There is a case where the personal computer is used as an audio server and the user downloads his favorite music pieces into a portable player and reproduces music. In the existing MD system, since the data is managed by using U-TOC, the affinity with the personal computers is insufficient. Therefore, for example, it is demanded to introduce a general management system such as an FAT system or the like and raise the affinity with the personal computers.
Since the audio data can be easily copied when the affinity with personal computers rises, necessity of protecting the copyright of the audio data increases. It is, therefore, demanded to encrypt the audio data and record it to thereby further enhance the protection of the copyright.
Further, a recording capacity of the disc of the existing MD system is equal to about 160 MB and such a capacity is no longer large. Therefore, it is demanded to increase the recording capacity while keeping the compatibility with the existing MD.